The invention relates to a system for tracking the edge of a magnetic medium or an optical medium as well as tracking the data thereon.
In high speed magnetic tape reading and writing units ("tape streamers"), the data is read from, or written in, a plurality of data tracks which run parallel to the edges of a magnetic tape. The write/read head of the system must, therefore, be accurately positioned over a selected track to read the data from the selected track or to write new data in the track. It is known, for example, from U.S. Pat. No. 4,476,503 to position the write/read head by first locating an edge of the tape, and then moving the head a specified distance from the edge to the desired track, the tracks being disposed on the tape at respective known distances from the edge.
It is a problem in this technology, however, that in practice the tape will not travel in a constant, perfectly straight path. The tape in moving will meander slightly and will follow an irregular serpentine path. The position of the edge must be constantly monitored with high accuracy and the position of the head is constantly adjusted through a feedback system because data tracks are usually packed so closely together on the tape that a small change in the edge position, in the absence of a corresponding change in the position of the head, can cause the head to be adjacent a track other than the desired track.
Various tracking systems are known in the art for many purposes, such as a transducer which may be positioned relative to a recording tape edge in an optical system. A light transmitter, such as a point source illuminating both the tape and a photo sensor, may be partly covered by the tape and placed in a fixed position relative to the light source and the transducer. The diameter of the photo sensor must be greater than the expected transversal range of movement of the tape. A control system allows the transducer to follow the transversal displacements of the tape.
Another known alternative embodiment implements a fixed position light emitting bar as the light source, and a photo sensor which is rectangular and of the same length as the light emitting bar and fixed to the transducer itself. This embodiment allows for the positioning of a recording head or a transducer relative to the tape edge for a multi-track recording system. Each position requires one reference input to the position controller of the transducer. This allows a signal proportional to the position of the transducer relative to the edge of the tape to be used as an input for the controller, which thereafter sends an error signal proportional to the difference between the reference and the output of the photo sensor to a motor which controls the position of the transducer. There are, however, several drawbacks to such a system. In a first version of the system, the transducer is normally placed between two tape guides and the light beam must be placed between one guide and the transducer itself. The problem is that the drift direction at the position of the light beam and at the position of the transducer can be different. A second version of this system corrects this drift problem; however, since the output from the photosensor is an analog signal, the system is subject to additional problems. The most severe problem is its sensitivity to small dust particles. With a magnetic medium, such particles settle on the illuminated part of the photodiode, and it is difficult to detect the occurrence of and to compensate for such noise. Updating of the DC output from the photodiode each time the transducer is placed in a given position is not possible. In addition, the system is sensitive to stray light pickup unless synchronous detection is used. Similarly, stray light pickup is difficult to compensate for unless the tape drive is completely shielded from external light sources. The use of an infrared source may help, but the stray light pickup is still a problem since infrared light may as well be present as background noise. In a dynamic tracking system, stray light pickup normally contains 100 Hz or 120 Hz components which will disturb a tape edge tracking servo unless synchronous detection is used. If an infrared light source is used, the photodiode may need a filter which is translucent for the wavelength used. This causes the distance from the tape to the photodiode to be increased which in turn reduces the sharpness of the transition zone between the light and dark area of the detector.
Another known method has a magnetic tape passing over a fixed recording/reading head which is automatically balanced in a vertical direction. The nominal vertical position of the tape is determined by at least one set of photo sensors and light emitting diodes and arranged such that the tape edge(s) partially covers the photo sensor(s). A typical arrangement embodies two sets of sensors, one for the lower edge and one for the upper edge of the tape. In this embodiment, the head is adjusted and fixed in a position which corresponds to equal outputs from the two sensors when the tape is placed in its nominal position. A control signal is obtained by simply taking the difference between the outputs from the sensors. The error signal is fed to a motor in a mechanical arrangement capable of adjusting the position of the tape. Such a system is susceptible to the same type of errors as discussed in the above system.
Another embodiment contains two sets of light emitters and receivers very similar to the one described in the preceding paragraph. Problems typical with an analog proportional system, such as difficulties with adjusting and maintaining equal light levels in the two emitters and a circuit for manually balancing or trimming the AC light levels and automatically the DC levels, are still present. The system is inherently susceptible to differences which may occur after the factory adjustments of the light in the two channels; such manual adjustment increases both the production and the component cost of the product.
An automatic track following system is also known which uses at least two separate detecting heads with read gaps wider than the written tracks and where the gaps have azimuth angles of equal values but of opposite rotational sign. During tracking, the centers of the azimuth head follows the centers of the corresponding two signal tracks. When the tracks drift away from the center positions of the azimuth heads, a lead/lag error signal can be extracted from the two heads if the information signal tracks contain some type of known synchronization, e.g. if video sync pulses have been recorded in parallel on both tracks. The polarity of the lead/lag signal determines the direction to move the head, and its value is proportional to the error if the tracks are located within the range of the azimuth gaps. Since auxiliary read gaps are used, responding only to the video sync pulses of long wavelengths, the azimuth angle can be tolerated. The extra tape noise from the unrecorded data can be tolerated in the timing channels due to the lower bandwidth requirement. However, the primary disadvantage of such a system is the inherent weakness of using azimuth heads for tracking, since such allows for a very limited linear tracking range. If tracking is disturbed, the control system has no information available about the direction to move the head. Noise pulses may cause head movement in the wrong direction as well. The head must be moved to the nominal position before the tracking system can be activated after a loss of the lead/lag signal, or a track seeking algorithm must be activated to start recovery. Another disadvantage is the added cost of the extra read heads. Such tracking systems are best suited for helical scan tape formats where the tracking can immediately lock on neighbor tracks if disturbed. Such a disturbance can be tolerated in some consumer analog video tape recorders (single frame loss or disturbance) or in helical scan data storage systems where interleaved data frames and error correction permits the loss of a track.
Another known embodiment proposes a two channel system for data recording where two azimuth read heads are used to derive the tracking error from the time skew between the read data clocks of the channels. The timing pulses are not so easily available as the sync pulses used in other prior art devices. However, this device does not require separate azimuth read heads, since the two write heads also have azimuth angles of opposite sign. The signals are either read back while writing by two aligned read heads with the same azimuth angles, or by the same write heads in simpler tape drives. An advantage of this system is greater information packing density, since no guard bands between tracks in the information area of the tape are needed. This device, however, is limited in the linear tracking range since it requires a very accurate openloop mechanical positioning mechanism in addition to the servo mechanism. An additional problem is the compensation or calibration of the time skew between the channels especially when reading tapes written in other drives. Yet another disadvantage is that if backward compatibility with older tape formats written without azimuth is to be maintained, at least one set of zero-azimuth read and write gaps must be provided. If one of the two write gaps is without an azimuth angle, half of the timing error is present as compared to a double azimuth scheme. The crosstalk from neighbor tracks will also increase.